ZynAddSubFX

ZynAddSubFX is a fully featured open source software synthesizer capable of making a countless number of instruments, from some common heard from expensive hardware to interesting sounds that you'll boost to an amazing universe of sounds. This cross-platform project is under active development

http://zynaddsubfx.sourceforge.net/

https://github.com/fundamental/zynaddsubfx

=Overview of Features:=
 * Polyphonic with support for legato and mono playing modes
 * Three synthesizer engines:
 * Additive Synthesis for classic synth sounds composed of a variety of voices with powerful modulation. This exposes modulators ranging from LFOs and envelopes to oscillator modulators for FM, PM, and AM.
 * Subtractive Synthesis for creating variable bandwidth harmonics from filtered white noise
 * PAD synthesis for creating beautiful pads and other instruments
 * Powerful waveform generator with up to 128 sine/non-sine harmonics
 * A variety of filters including analogue modeled filters, formant filters, and state variable filters.
 * Envelopes can have ADSR (or ASR, etc..) modes or can be free modes (with any shape)
 * Effects for Reverb, Echo, Chorus/Flange, Phasing, Wave-shaping, Equalizing, Dynamic Filtering with flexible signal routing
 * Instruments can be organized in kits, which allows you to make drum kits or layered instruments; this makes possible to use more than one instrument for a single part. It is possible to choose what items from * the kit should be processed by the Part's effects.
 * Randomness settings to create subtle differences in each sound to help create that familiar analogue warmth.
 * Microtonal capabilities with any scale, up to 128 notes per octave, and key mapping
 * Extensive MIDI/Audio driver support including JACK, ALSA, OSS, and PortAudio
 * A built-in Virtual Keyboard which can be used even if you don't have a physical midi keyboard
 * The graphical user interface can be completely disabled if the user need so
 * Session Management Support via LASH/NSM
 * Plugin Support via DSSI/LV2/VST
 * Over 1100 high quality instruments included